In a wireless network, the quality of the service (QoS) is an important factor of user satisfaction. In order to assure high QoS the network must provide sufficiently high throughput per user. This can be in part achieved by ensuring the limited resource of wireless spectrum is efficiently utilized. Techniques such as TMDA, FDMA, CDMA, OFDMA, CSMA allow effectively simultaneous communication of a multitude of mobile stations (MS) with the base station.
However, to further increase the throughput per user once the spectrum is efficiently utilized, the network must be divided into geographically separate segments or cells, each being served by its own base station. A single cell serves local MS only; by having smaller cells, less MSs per cell share the same bandwidth. Thus, each MS is granted a higher share of the available bandwidth.
To maximize net throughput per user in 4G networks, an operator utilizes a large number of cells much smaller than cells in traditional cellular networks. Such cells are termed pico cells, each served by a pico base station.
In a network, covered by a high number of densely distributed base stations with partially or completely overlapping cells, it is crucial to automate the network operation and management. An important aspect of network management is congestion avoidance, which assures that no single base station is overloaded if there is available transmission capacity available that could be used to offload the congested base station. However, such high density of 4G networks at the same time increases the amount of handoff (handover) events, where mobile station switches from one base station to the next.
Handoffs typically occur when a mobile station moves between two cells, when environmental conditions affecting radio wave propagation change or when base station becomes congested. In the best case, successful handoffs introduce communication overhead. However, in the worst case unsuccessful handoffs can degrade communication performance and can cause the connection to drop. Ensuring handoff success thereby represents an important part of assuring acceptable QoS.
A multitude of methods concerning handoff optimization exist. A method described in U.S. Pat. No. 7,379,739 relies on a handoff controller which monitors MS location and velocity and uses that information as a basis for optimum base station estimation. Furthermore, GB 2370449 A similarly discloses a method incorporating a kinetic unit for determining kinetic characteristics (speed, location) of the MS and BS signal strength to determine optimal target cells for handoff. US patent application No. 2008/0240043 A1 discloses a method for performing handoff considering QoS in a broadband mobile system; the method is based on a MS performing the signal strength measurements and making a handoff request when better conditions are available at a different BS. Similar approaches, based on RSSI measurements on the MS are described in patent applications US 2009/0005052 A1 and US 2009/0275335 A1.
The fact that in a 4G network each base station advertises its local neighbors to the mobile stations enables flexible high-level handoff management that was not possible in traditional cellular networks.
The present invention aims to orchestrate handoff events of the 4G network, thus reducing the number of necessary handoffs of a MS. The 4G Self-Organizing Network (SON) server performs a neighbor selection procedure which determines a limited set of optimal handoff candidates and excludes the neighbors with low handoff success probabilities. The SON server can additionally apply machine learning algorithms to the historic handoff data to further optimize the vicinity area which serves as a starting point for completely automated optimum neighbor selection.